The present invention relates to a distance-measuring instrument mounted in an automobile to detect a distance to a front vehicle.
A conventional distance-measuring instrument for detecting a distance comprises image pickup means composed of a pair of image-forming means and photo sensor arrays; and arithmetic means using two images of a measured object photographed by the image pickup means to calculate the distance to the object based on the principle of triangulation.
First, the principle of triangulation is described in brief with reference to FIG. 8.
Image-forming lenses 51a and 51b form images 23 and 24 of an object on photo sensor arrays 25 and 26. Since triangles 27 and 28 are similar to triangles 27' and 28', respectively, the distance L to the object is expressed by Equation 1. EQU L=Bf/(x1+x2)=Bf/x
The inter-optical-axis distance (hereafter referred to as the "base length") B between the image-forming lenses 51a and 51b and the focal length (f) are constant, so the distance L to the object can be determined by detecting (x).
FIG. 9 shows a sectional view of a conventional distance-measuring instrument formed based on the principle of triangulation. This distance-measuring instrument consists of the image-forming lenses 51a and 51b, a lens-holding member 52, and CCD packages 53a and 53b which act as photosensor arrays, and a CCD holding member 54.
The image-forming lenses 51a and 51b are formed of polycarbonate plastic lenses having low water absorption and excellent optical characteristics. The CCD packages 53a and 53b are formed of ceramics, and the lens holding member 52 and the CCD holding member 54 are formed of aluminum die cast products having low thermal expansion so as to maintain locational accuracy between the lenses and between the CCDs.
In particular, the lens holding member 52 and the CCD holding member 54 are formed of the same material, that is, aluminum. Therefore, when they are thermally expanded, they advantageously contract equivalently only if a (UV+heat)-setting adhesive 55 is used to fix them together.
To join the image-forming lenses 51a and 51b and the lens-holding member 52 together, however, the (UV+heat)-setting adhesive 55 for fixing the positions and a thermosetting adhesive for maintaining the adhesive strength are used.
This is because the application of the (UV+heat)-setting adhesive 55 all over the lens surface allows the lens to be fixed firmly, but causes stress when the lens and the holding member 52 are heated due to the firm fixation on all the surface, causing aluminum to peel off the plastic surface due to the difference in linear expansion coefficient.
Thus, two adhesives must be used such that the (UV+heat)-setting adhesive 55 firmly fixes only one point of the lens, that is, the boss 51c, while a thermosetting adhesive 56 is used for most of the lens surface to provide good adhesive strength.
On the other hand, due to the difference in linear expansion coefficient between the ceramics and aluminum, direct connection of the CCD packages 53a and 53b and the CCD holding member 54 together causes thermal stress when they are expanded and contracted. Thus, an iron piece 57 having an intermediate linear expansion coefficient is placed between the packages and the holding member, and the (UV+heat)-setting adhesive 55 is used to fix them.
This reduces the thermal stress between these members due to changes in ambient temperature, and thus prevents peeling.
A breathing filter 59 in FIG. 9 discharges moisture in the enclosure to the exterior to equalize the humidity between the interior and exterior of the enclosure.
Even if, however, both the lens-holding member 52 and the CCD-holding member 54 are formed by an aluminum die cast product, both the CCD packages 53a and 53b and the image-forming lenses 51a and 51b can not be formed of the same material, and the thermal stress between the joined portions of the different types of materials can not be totally eliminated due to the difference in linear expansion coefficient.
For example, a silicone thermosetting adhesive 56 is used to join the plastic image-forming lenses 51a and 51b and the lens-holding member 52 of an aluminum material, and this silicone material acts as rubber particularly at a hot portion in order to prevent aluminum from being peeled off from plastics.
When the silicone thermosetting adhesive is thermally expanded at a hot portion, however, aluminum and plastics maintain the adhesive strength but do not return accurately to their original positions when contracted, resulting in a relative offset of 5 .mu.m for the base length between the CCDS.
Based on the principle of triangulation, this offset of 5 .mu.m corresponds to a distance-measuring error of about 4 m (relative to a true value L=30 m). Accordingly, even a minor offset between the materials significantly affects the distance measurement accuracy.
FIG. 10 shows the difference in locations at which the CCDs are adhered.
If the CCD packages 53a and 53b are fixed in such a way that their centers are spaced by the base length B as in FIG. 10(a), the thermal expansion between the CCDs only corresponds to the base length B of the CCD-holding member 54 and equals the thermal expansion between the image-forming lenses. Consequently, no error occurs in distance measurements.
If, however, for example, the CCD packages are fixed in such a way that the centers of the adhesives are spaced by a distance shorter than the base length as in FIG. 10(b), the thermal expansion between the CCDs decreases accordingly, and this results in a difference between this distance and the base length between the lenses. Reference B' designates the distance between the centers of the adhesives (B&gt;B').
Since the surfaces of the CCDs and the CCD-holding member are adhered together via the iron piece 57, the centers of the adhesives do not necessarily align with the centers of the surfaces.
This is due to variation during the application of the adhesives (amount of adhesive applied, and amount and direction of pressure applied).
Thus, the locations of the centers of the adhesives may vary by .+-.5 mm at maximum.
The difference in expansion caused by this variation substantially affects the device, particularly on a hot side.
Furthermore, the thermal conductivities of aluminum and plastics differ in nearly 100 times. Since the inside of an automobile is subjected to hot and cold atmospheres repeated, a large amount of time is required until the temperature inside the image pickup module stabilizes in all over the module. Accordingly, there is often a difference between the base length between the image-forming lenses and the base length between the CCDs.
Even if an aluminum member having low thermal expansion is used for the enclosure portion, the materials of the CCD packages and image-forming lenses are limited, so that the thermal stress between the different types of materials in these portions can not be totally eliminated.
In addition, when the CCD holding member 54 and the lens holding member 52 are formed of an aluminum die cast material and the CCD packages 53a and 53b are formed of ceramics, these members are relatively expensive. The assembly process for such members is also complicated and expensive due to the difficulty in achieving good adhesion and the large number of required parts.
This structure reduces the difference in linear expansion coefficient between the different types of materials when it is compared with the structure in which the CCD-holding member 54 and the lens-holding member 52 are formed of plastics while the CCD packages 53a and 53b are formed of ceramics. Even in this structure, however, the adhered portions may be displaced or peeled off.
Furthermore, even if the CCD packages 53a and 53b and the CCD holding member 54 are composed of plastics as shown in FIG. 11, when the CCD packages 53a and 53b are adhered and joined to the CCD-holding member 54, the direct transmission of heat from CCD chips 58a and 58b may cause the CCD-holding member to expand noticeably.
When power is turned on, there will be a difference in temperature of about 10.degree. C. between the CCD-holding member and the lens-holding member, which causes a difference in the base length of about 30 .mu.m.
Consequently, the difference in the base length between these members may result in erroneous distance measurement.
Besides the heat from the CCD chips 58a and 58b, if a D/D converter is mounted on a CCD circuit board to which the CCD packages 53a and 53b are soldered, heat from this heat source is transmitted to the CCD-holding member 54 causing it to expand and again leading to erroneous distance measurement.
As described above, if one of the image-forming lens, the lens-holding member, and CCD-holding member is composed of a different material, there will be large differences in the base length between the image-forming lenses and the base length between the CCDs.
Ideally, all members are formed of plastics. Since, however, conventional CCD packages 53a and 53b made of plastics such as those shown in FIG. 11 are normally manufactured by injection-molding an acrylic martial, they are subjected to variation in sizes and internal condensation caused by moisture absorption.
Thus, even if the image-forming lenses, the lens-holding member and the CCD-holding member are all formed of the same material (plastics), the conventional techniques still have many practical problems.
It is thus a main object of this invention to provide a distance-measuring instrument whose size does not vary with moisture absorption even if the image-forming lenses, lens-holding member and photo sensor (CCD) array-holding member are all formed of the same plastic material.
It is another object of this invention to provide a distance-measuring instrument whose distance-measurement accuracy is maintained in all temperature ranges based on the fact that the linear expansion coefficient of the components are equal.
It is yet another object of this invention to provide a distance-measuring instrument that can be manufactured inexpensively.